1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor and a method of producing it. More particularly, the present invention relates to a monolithic ceramic capacitor having dielectric ceramic layers each made of a ceramic material comprising strontium titanate as the main component and bismuth oxide or an oxide of a bismuth compound as an additional component and containing a reduction inhibitor, and also to a method of producing it.
2. Description of Related Art
A monolithic ceramic capacitor is generally composed of a plurality of dielectric ceramic layers with inner electrodes sandwiched between the adjacent ceramic layers and with outer electrodes disposed on the outer surface of the laminate and connected with each inner electrode.
Such a monolithic ceramic capacitor may be produced according to the following process. A dielectric powder is mixed with an organic binder; the resulting mixture is made into a slurry; the slurry is shaped into green sheets by a doctor-blading method; the sheets each are printed with a paste containing a metal powder by a screen-printing method or the like; a plurality of the green sheets each printed with the paste are laminated; the resulting laminate is subjected to pressure to establish tight connections between the green sheets; the laminate is then fired in air at from about 1300 to 1400xc2x0 C.; an outer electrode material is applied onto the surfaces of the sintered body having exposed inner electrodes; and the material is baked.
Dielectric ceramic layers consisting essentially of strontium titanate and containing a bismuth compound give the monolithic ceramic capacitor a relatively large dielectric constant with small voltage dependence and a small dielectric loss and therefore high resistance to high voltage.
A material of the inner electrodes comprising a noble metal such as platinum, gold, palladium or their alloys is advantageous in that (1) the inner electrode material does not react with the dielectric ceramic during firing since the noble metal has a melting point higher than the temperature at which the dielectric ceramic is fired and that (2) the inner electrode material is not oxidized even when fired in air. However, an electrode material comprising such a noble metal is expensive. For example, the cost of the electrode material may often be from 30 to 70% of the total material cost of the ceramic capacitor.
Moreover, an inner electrode material comprising a silver-palladium alloy often worsens the characteristics of the capacitor. For example, the dielectric constant of the capacitor is often lowered and becomes unstable due to the migration of silver.
In order to solve these problems, base metals such as nickel, copper, iron, cobalt, tungsten, molybdenum, etc. could be used as the inner electrode material. However, the material comprising such a base metal must be fired in a neutral or reducing atmosphere in order to prevent the oxidation of the material during firing. If fired in such conditions, the bismuth compound in the dielectric ceramic layers is reduced and, as a result, the dielectric ceramic becomes semi-conductive. Accordingly, a ceramic capacitor would not be produced.
Copper can be fired at about 1000xc2x0 C. in a neutral atmosphere having an oxygen partial pressure of 10xe2x88x927 MPa but it is easily oxidized at relatively low temperatures. An inner electrode material comprising copper also diffuses copper into the dielectric ceramic layers during firing at an oxygen partial pressure that is higher than the equilibrated oxygen partial pressure of Cu/CuO and often lowers the characteristics of the capacitor produced. Therefore, an inner electrode material comprising copper requires the strict control of the oxygen partial pressure in the firing atmosphere, which, however, results in the increase in the production cost.
The present invention provides a monolithic ceramic capacitor which comprises a dielectric ceramic material consisting of strontium titanate as the main component and bismuth oxide or an oxide of a bismuth compound as an additional component and an inner electrode material of a base metal and which therefore has a large dielectric constant with small voltage dependence and a small dielectric loss and has high resistance to high voltage.
Specifically, as the first aspect of the present invention, there is provided a monolithic ceramic capacitor which is composed of dielectric ceramic layers made of a ceramic material comprising strontium titanate as the main component and bismuth oxide or an oxide of a bismuth compound as an additional component and containing a reduction inhibitor, and inner electrodes made of a base metal material containing nickel or a nickel alloy.
As one preferred embodiment of the first aspect of the invention, the reduction inhibitor in the ceramic material is represented by a general formula:
aMO+bMnO2+cB2O3+(100xe2x88x92axe2x88x92bxe2x88x92c)SiO2
wherein M is at least one of Mg, Sr, Ca and Ba; and a, b and c are 10xe2x89xa6axe2x89xa660, 5xe2x89xa6bxe2x89xa620 and 20xe2x89xa6cxe2x89xa635 in mol percent.
As another preferred embodiment, the reduction inhibitor is from about 4 to 25% by weight relative to the ceramic material containing it.
The present invention also provides as its second aspect a method for producing a monolithic ceramic capacitor, which comprises the steps of preparing dielectric ceramic green sheets and laminating an electrode material on each green sheet to form a laminate comprising the ceramic green sheets each with the electrode material, followed by heating the resulting laminate at a rate to realize an increase in the temperature of the laminate of from about 10 to 17xc2x0 C./min, firing it at a predetermined temperature and thereafter cooling it.
As one preferred embodiment of the second aspect of the invention, the laminate is cooled at a rate of about 10xc2x0 C./min or more in the last cooling step.
As another preferred embodiment, the ceramic green sheets prepared comprise strontium titanate as a main component and bismuth oxide or an oxide of a bismuth compound as a minor component and contain a reduction inhibitor as an additive.
As still another preferred embodiment, the electrode material laminated to be on each ceramic green sheet comprises a base metal material containing nickel or a nickel alloy.
As still another preferred embodiment, the reduction inhibitor in each ceramic green sheet is represented by a general formula:
aMO+bMnO2+cB2O3+(100xe2x88x92axe2x88x92bxe2x88x92c)SiO2
wherein M is at least one of Mg, Sr, Ca and Ba; and a, b and c are 10xe2x89xa6axe2x89xa660, 5xe2x89xa6bxe2x89xa620 and 20xe2x89xa6cxe2x89xa635 in % by mol.
The preferred reduction inhibitor is of the general formula:
aMO+bMnO2+cB2O3+(100xe2x88x92axe2x88x92bxe2x88x92c)SiO2
wherein M is at least one of Mg, Sr, Ca and Ba; and a, b and c each are % by mol. It contains a reduction inhibitor MO. If MO is less than about 10 mol % of the reduction inhibitor, the insulating resistance of the ceramic capacitor is lowered and the dielectric loss thereof is increased. If it is more than about 60 mol %, the insulating resistance of the ceramic capacitor is also lowered. Therefore, the mol % (a) of MO in the reduction inhibitor preferably satisfies 10xe2x89xa6axe2x89xa660 and most preferably 35xe2x89xa6axe2x89xa655.
Also, MnO2 functions as a reduction inhibitor. If b is less than about 5 mol %, the dielectric ceramic becomes semi-conductive. If it is more than about 20 mol %, the insulating resistance of the ceramic capacitor is lowered. Therefore, it is desirable that 5xe2x89xa6bxe2x89xa620 and most preferably 10xe2x89xa6bxe2x89xa615.
B2O3 functions as a glass former. If c is less than about 20 mol %, the green sheets cannot be sintered sufficiently. Therefore, the dielectric loss of the ceramic capacitor is increased and the insulating resistance thereof is lowered. If c is more than about 35 mol %, the dielectric constant of the ceramic capacitor is lowered. Therefore, it is desirable that 20xe2x89xa6cxe2x89xa635 and most preferably 25xe2x89xa6cxe2x89xa630.
SiO2 functions as a glass former. If a, b and c are outside the ranges of 10xe2x89xa6axe2x89xa660, 5xe2x89xa6bxe2x89xa620 and 20xe2x89xa6cxe2x89xa635, the insulating resistance of the ceramic capacitor is lowered, the dielectric loss thereof is increased and the dielectric constant thereof is lowered.
If the amount of the reduction inhibitor added to the dielectric ceramic material is less than about 4% by weight, it is difficult to prevent reduction of the dielectric material. If, however, it is more than about 25% by weight, the dielectric constant of the ceramic capacitor is noticeably lowered. Therefore, it is preferable that the monolithic ceramic capacitor of the present invention contains from about 4 to 25% by weight of the reduction inhibitor relative to the dielectric ceramic material and more preferably about 8 to 20%.
The firing temperature of the ceramic is conventional. However, in the method of the present invention for producing the dielectric ceramic, when the heating rate to firing temperature is lower than about 10xc2x0 C./min, the dielectric ceramic becomes semi-conductive. If, however, it is higher than about 17xc2x0 C./min, the green sheets cannot be sintered sufficiently. If the heating rate is lower than about 10xc2x0 C./min and the cooling rate for cooling the sintered body is lower than about 10xc2x0/min, the dielectric ceramic also becomes semi-conductive. Therefore, in the method of the present invention for producing the dielectric ceramic, it is preferable that the heating rate for heating the laminate is at from about 10 to 17xc2x0 C./min, preferably about 15 to 17xc2x0 C./min, and that the cooling rate for cooling the sintered body is about 10xc2x0 C./min or higher, preferably about 15 to 17xc2x0 C./min.
Since a dielectric ceramic material containing a reduction inhibitor is used to produce the monolithic ceramic capacitor of the present invention, the bismuth compound in the dielectric ceramic material is not reduced even when the material is fired in a neutral or reducing atmosphere. According to the present invention, it is therefore possible to obtain a monolithic ceramic capacitor having a high dielectric constant with small voltage dependence and having a low dielectric loss.
In addition, since the monolithic ceramic capacitor of the present invention can be produced by firing in a neutral or reducing atmosphere, an inner electrode material comprising nickel or a nickel alloy can be used and is not oxidized. The nickel does not migrate like silver does when used to make inner electrodes.
Accordingly, the present invention can provide monolithic ceramic capacitors with excellent characteristics at low cost.
In the method of the present invention for producing a monolithic ceramic capacitor, the heating rate for heating the ceramic laminate is from about 10 to 17xc2x0 C./min and the cooling rate for cooling the sintered ceramic body is about 10xc2x0 C./min or higher. Therefore, the method can produce monolithic ceramic capacitors with excellent characteristics while preventing the inner electrodes from being oxidized and preventing the dielectric ceramic layers from becoming semi-conductive.
In addition, even when nickel or a nickel alloy is used as the inner electrode material in the method, the nickel is not oxidized when firing under an oxygen partial pressure higher than the equilibrated oxygen partial pressure of Ni/NiO.